Recently, an image encoding method having a greatly increased encoding efficiency has been recommended as ITU-T Rec. H.264 and ISO/IEC 14496-10 (referred to as H.264 hereinafter) through the cooperation of ITU-T and ISO/IEC. In H.264, a prediction process, conversion process, and entropy encoding process are performed for each rectangular block (for example, each block of 16×16 or 8×8 pixels). In the prediction process, motion compensation of performing prediction in the temporal domain is performed on a rectangular block to be encoded (an encoding target block) by referring to an already encoded frame (reference frame). In this motion compensation, it is necessary to encode motion information containing a motion vector as spatial shift information between the encoding target block and a block to be referred to in the reference frame, and transmit the encoded motion information to a decoding side. When performing motion compensation by using a plurality of reference frames, it is also necessary to encode reference frame numbers in addition to the motion information. This sometimes increases the code amount of the motion information and reference frame numbers. There are also motion information prediction methods by which predicted motion information of an encoding target block is derived by referring to motion information of a reference frame, which is stored in a motion information memory (Japanese Patent No. 4020789 and Yeping Su et al, “CE9: Reduced Resolution Storage of Motion Vector Data”, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11 Document, JCTVC-D072, January 2011), and the capacity of the motion information memory for storing motion information sometimes increases.
As an example of a method of reducing the capacity of the motion information memory, representative motion information is derived in a predetermined block and only the representative motion information is stored in the motion information memory in (Yeping Su et al, “CE9: Reduced Resolution Storage of Motion Vector Data”, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11 Document, JCTVC-D072, January 2011).